BULLETIN 231] WALNUT CULTURE IN CALIFORNIA. 373 



down to the main forks of the tree or the tree may die outright clear to 

 the ground. The former type of die-back, affecting old trees slowly, 

 occurs mostly on light soils where the roots find insufficient moisture as 

 they become crowded together and extend down into a dry, sandy sub- 

 soil. The second form of die-back mentioned, that occurring in trees 

 which have formerly been thrifty, killing them down to the forks or 

 even to the ground, is sometimes an effect of frost, but occurs especially 

 with or without freezing in cases where the ground has become too dry 

 during the fall and winter. This is of quite common occurrence in 

 walnut groves, especially in certain seasons when the rainfall is deficient 

 during the fall and early winter. At this time of year irrigation of 

 dormant deciduous trees does not seem to be urgent and serious damage 

 sometimes occurs. In a cold locality or season this damage may be very 

 much increased by frost, when, if the soil had been sufficiently moist, no 

 die-back would have occurred. The remedy is therefore obvious, and it 

 is significant that fall and winter irrigation of walnut groves is coming 

 much more into practice every year. It may be said in general that 

 almost all die-back of walnut trees is connected with the matter of soil 

 moisture. Usually it is the direct result of a dry subsoil, although in 

 old, closely planted groves the trouble may be increased by a lack of 

 proper distance between the trees, deficiency of plant food, and similar 

 factors. 



Instances are sometimes seen when the opposite condition prevails, 

 die-back being caused by an excess of moisture in the subsoil, a condi- 

 tion of which the walnut tree is extremely intolerant. Such damage 

 may be seen to a marked extent in some of the coastward regions of 

 Ventura County and in other parts of the State, where, on account 

 of floods or other unusual conditions, the ground water has risen in 

 walnut groves close to the surface. 



SUNBURN BLACK SAP. 



Serious damage is sometimes caused from this source, both to the 

 fruit and the tree. This we have already discussed to some extent on 

 page 177, in considering the relation of climatic conditions to walnut 

 culture. The sudden occurrence of extremely hot, sunny weather 

 when the nuts are nearly full grown may produce a blackening and 

 burning of the husk on one side, causing the husk to stick to the shell, 

 making separation difficult and causing a black spot which may extend 

 through into the meat. Continuous hot weather may cause a darkening 

 or brownish color of the meat which would otherwise be nearly white, 

 without a burning of the outer husk. This varies to a large extent in 

 different varieties. 



